This invention concerns alarm devices which are able to detect differences in acoustic pressure following the untimely opening or break-in of a door or window and specifically concerns a self-adjusting alarm device with low power consumption.
In alarm devices of this type, the output signal from a microphone is first amplified, then, generally, compared to a reference voltage set in a comparator whose output may have two possible states depending on the relative value of the signal received from the microphone and the reference voltage.
These devices trip the alarm as a result of an aperiodic compressional wave, although they remain in sensitive to a periodic signal such as an audible sound, with monitoring of namely the shape and amplitude of the signals received.
In the majority of devices of previous art designed to warn of the untimely opening of doors and windows in a closed room, threshold sensitivity must be adjusted manually on a case by case basis.
In practice, this adjustment is closely linked to possible sealing defects of the site concerned, as well as to the excessive flexibility of certain construction materials used, which, in the event of strong winds, give rise to pressure variations inside the room as a result of wind surge or air infiltration.
In order to do away with the alarm tripping which is not caused by a break-in, the sensitivity threshold of these detectors is set relatively high so that they do not react to such random and fugitive atmospheric disturbances which are unavoidable as they are the result of strong winds. This type of adjustment may reduce the detector""s efficiency during calm weather.
In order to counter these drawbacks, the applicant developed a self-regulating alarm device described in European patent 0.317.459. In this device, an acoustic pressure differential detector features a sensitivity threshold which is constantly adjusted to its optimal value by the microphone""s output signal which varies in accordance with the atmospheric disturbances that are detected at the microphone input.
Unfortunately, the device described in patent EP 0.317.459 relies upon analog electronic components such as capacitors and resistors whose characteristics vary from one component to another for the same type of component. This deviation in the characteristics for a given component, even if it is relatively low, may result in significant operational variations between two devices insofar as the operation of the device is based on the combination of a plurality of such components. In addition, this type of device generally has a permanent power supply and thus leads to excessive energy consumption due to the fact that it is connected to the mains in a wired alarm central station.
This is why the purpose of the invention is to supply self-adjusting alarm devices having insignificant operating variations from one device to another owing particularly to the fact that part of the device""s functions is performed by a microprocessor.
Another purpose of the invention is to supply an alarm device of the type above having very low energy consumption owing to the use of a microprocessor.
Consequently, the invention concerns an alarm device featuring an acoustic pressure sensor supplying an analog signal to a first amplifying means and to a second amplifying means, a first comparator whereof the +input is connected to the output of the second amplifying means and whereof the output delivers a warning signal to alarm means when there is an actual break-in or a break-in attempt. This device includes self-adjusting means consisting mainly of an analog-digital converter, the input of which is connected to the output of the first amplifying means in order to supply at the output a digital signal that varies in accordance with the atmospheric disturbance and a microprocessor programmed to deliver, in response to the detection of the digital signal supplied by the converter, a digital signal at the xe2x88x92input of the comparator, whereof the pulses have a variable width which increases in accordance with the duration and the importance of the atmospheric disturbance so as to automatically increase the alarm device""s triggering threshold and hence reduce its sensitivity when the acoustic sensor detects an atmospheric disturbance such as wind.